Unit cooler with integrated refrigeration and dehumidification

ABSTRACT

An integrated refrigeration and dehumidification unit cooler for a refrigerated space, such as a cold room, includes directly serially connected refrigeration and heating coils operationally interfaced to effect passive control of room temperature and humidity conditions preventing overhead condensation.

FIELD OF THE INVENTION

The present invention relates to systems for conditioning enclosed spaces, and in particular to a unit cooler for refrigerating and dehumidifying a storage space and for eliminating moisture condensation therein.

BACKGROUND OF THE INVENTION

Present governmental code, regulations, and guidelines do not allow any water to drip from overhead surfaces in food processing rooms down upon the production area. Additionally, those regulations have strict limits as to the temperatures at which refrigerated food must be maintained, 41° F. being the accepted upper limit. For decades, food processors have utilized traditional direct expansion refrigeration unit coolers as standard components in field erected, or built up, systems that refrigerate the processing rooms. The unit coolers are located within the processing room and include a cooling coil and fan for circulating and cooling the air within the room. The cooling coil is connected at a direct expansion device interior to the unit cooler and to a compressor located exterior of the processing room. Installation requires only two fluid connections and modest control.

These unit coolers are effective at temperature control, but not at humidity control. The direct refrigeration system maintains the relative humidity (% Rh) levels in the food processing room at about 90 to 100% Rh. These high Rh. levels present problems for food processors. From time to time, the overhead equipment and/or the overhead structure gets colder than the dew point of the air in the room, particularly during defrost cycles. In these situations, water from the air condenses on the overhead equipment and/or the overhead structure. The condensed water then drips down upon the production area. Such dripping raises the potential for food contamination. If a governmental or private inspector notes such dripping from the ceiling, they will typically require a cleanup and sanitation procedure in the food processing room. This shutdown results in waste disposal of the involved food and many labor hours of lost production time for the food processor. As a result this is an active interest in eliminating condensation through improved dehumidification.

One basic approach is currently used, wherein a separate desiccant dehumidification system is added to supplement the conventional unit cooler. While satisfactory for avoiding condensation problems, installation and equipment costs are high and a large increase in energy consumption, 50% to 200%, is incurred because the dehumidifiers use energy to operate and add heat to the refrigerated space. This in turn increases the cooling load on the refrigeration system and results in higher energy cost for the refrigeration system.

It would be desirable to provide a unit cooler for these cold room facilities that would provide both the requisite refrigeration and operate at humidity levels overcoming condensation problems. The use of serial refrigeration and reheat coils has been proposed for lowering excessive humidity conditions in ambient personal comfort conditions. Representative of such an approach is disclosed in U.S. Pat. Nos. 5,622,057 and 3,798,920 wherein a reheat coil, modulated in response to humidity conditions, is inserted fluidly before the cooling coil and downstream thereof in the airflow. The two coils are substantially the same. When dehumidification is required, the subcooling coil is operative until set conditions are attained and thereafter modulates within control limits. Such systems are accepted for applications above about 50° F., but are not approved by the manufacturers for high humidity conditions below this temperature. For temperatures below this level, the coil must operate at below freezing temperatures, resulting in progressive ice buildup requiring defrost cycles and varying the air flow to the reheat coil. This results in control instability leading to high maintenance costs, compressor failures, poor room temperature control, erratic air flow and poor room humidity control.

SUMMARY OF THE INVENTION

This present invention provides a unit cooler for cold rooms that passively provides temperature and humidity control without modulation. The unit cooler provides full time dehumidification and refrigeration in a factory assembled unit cooler that may be installed and operated without complexity for either the system installer or the system operator. The unit cooler uses directly and continuously serially connected refrigeration direct expansion (DX) and reheat/subcool (SR) coils to achieve lower % Rh room conditions without a large increase in energy consumption.

The foregoing advantages are achieved by a unit cooler employing design criteria that allows the refrigeration system to dehumidify and run through defrost cycles without additional controls or complexity or the difficulties of the prior art systems discussed above. The unit cooler provides a passive and natural balance of over-cool and reheat of air during refrigeration. This is achieved by providing a desired dew point for the supply air that avoids condensation, operating the DX coil at a temperature is achieve the dew point and operating the SR coil to achieve the desired supply temperature for maintaining the space at room design temperature.

In achieving this benefit, the DX coil is operated at a reduced saturated suction temperature (SST) about 3 to 9 degrees F. without significantly increasing compressor equipment or operating cost. By example, if a conventional system operates at +25° F. SST as a traditional DX system, the design suction temperature is lowered to ±20° F. SST. This represents an efficiency loss to the system of roughly 8%, the traditional DX system Energy Efficiency Ration (EER) being about 11.2, the present invention operates at EER of about 9.2, which is significantly recoupled in the present invention.

The unit cooler design criteria also requires that the SR coil gives energy back to the refrigeration system that has experienced a loss in design efficiency due to the above decrease in design SST. That energy is returned in the SR coil pass as the condensed liquid is refrigerated from saturation to a subcooled state, and allows for about 15 to 25% of compressor cooling capacity to be returned in liquid cooling. To accomplish this, the unit cooler is to be designed with the SR coil to have 15% to 35%, and preferably 20% to 30% as much primary coil surface and secondary coil surface as the DX coil. Above and below this range, passive operation cannot be attained and unreliable modulating operation would be required. This recovers much of the lost efficiency in the system, resulting in a system having an EER of only 3% or less than the traditional DX refrigeration system, and substantially less than the energy costs of separate units for dehumidification.

Generally it is preferred to deliver air to the cold room at roughly the same temperature, i.e. 41° F. or less, and flow rate as it would with a traditional DX refrigeration system to maintain the design temperature. This is accomplished because the 3 to 9° F. lower design SST creates a minimum air temperature at the DX coil that is about 2 to 8° F. lower than the tradition DX refrigeration system. This establishes a dew point with the passively integrated unit cooler that is about 2 to 8° F. lower than a traditional DX refrigeration system.

With these design criteria the unit cooler naturally delivers air with a relative humidity at 70% to 85%. For operation within this range, controls are not needed as the dehumidification unit cooler naturally balances out at these conditions. With this supply air relative humidity, the passively integrated unit cooler prevents water condensation dripping issues in the refrigerated space.

The ability to refrigerate and dehumidify at the same time, this invention has many applications outside of food processing rooms. Those applications include controlled humidity storage rooms for goods ranging from seeds to paper and from consumer goods to industrial goods.

Accordingly, it is an object of the present invention to provide a unit cooler providing a supply air at a humidity avoiding condensation in cold room facilities.

Another object is to provide a unit cooler refrigeration system that passively provides humidity control for cold room facilities.

A further object is to provide a unit cooler for preventing condensation problems in cold room facilities that may be installed and operated without increased complexity.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of the present invention will become apparent upon reading the following written description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic drawing of cold room refrigeration system having a unit cooler with integrated refrigeration and dehumidification in accordance with an embodiment of the invention;

FIG. 2 is a schematic drawing of the unit cooler of FIG. 1;

FIG. 3 is a schematic drawing of a cold room refrigeration system incorporating a unit cooler in accordance with the prior art;

FIG. 4 is a schematic drawing of the unit cooler of FIG. 3;

FIG. 5 is a pressure/enthalpy diagram comparing the systems of FIG. 1 and FIG. 3;

FIG. 6 is a psychometric chart of the prior art refrigeration system of FIG. 3; and

FIG. 7 is a psychometric chart of the refrigeration system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A traditional DX refrigeration system for a unit cooler for conditioning of a refrigerated space such as a cold room or walk-in refrigerated or freezer room is shown in FIG. 3. Therein, a DX refrigeration system 100 is provided for controlling the interior temperature of a refrigerated room 102. The refrigeration system 100 comprises a refrigeration compressor 110 that delivers working fluid such as refrigerant as a hot, high pressure discharge gas from an outlet through outlet line 112, to the inlet of a condenser 114. The condenser 114 changes the refrigerant from a hot gas to a warm liquid by rejecting heat from the refrigerant into another medium such as air or water. The warm liquid is conveyed from an outlet of the condenser through refrigerant line 116 to the inlet of a direct expansion device 118 and through line 120 to a unit cooler 122 located within the room 102. As shown in FIG. 4, the unit cooler 122 includes a housing 124 typically mounted at the ceiling area 125 of the room. The housing 124 encloses refrigeration DX coil 126. The expansion device 118 expands the refrigerant from a warm, high pressure liquid to a cold, low pressure mixture of liquid and vapor refrigerant. This mixture is delivered from the outlet of the expansion device 118 to the inlet of the DX coil 126. The unit cooler 122 includes a fan 128 for drawing return air from an inlet 130 through the coil 126 to an outlet 132 for circulating conditioned air to the room at a saturated controlled temperature, as indicated by the arrows. The unit cooler 122 thus refrigerates the room 102 by absorbing heat at the coil, boiling most or all of the cold liquid refrigerant to cold vapor refrigerant. The resulting cold vapor is conveyed at low pressure from the outlet of the coil 126 through line 134 to the suction side or inlet of the compressor 110 at a saturated suction temperature (SST).

The installation is simple requiring only two fluid connections at the preassembled unit cooler The controls for such a system are also basic, requiring a temperature control, and a defrost system for deicing the coil. Deicing may be effected, during system shut down conventionally through ambient thawing, or electric or gas assisted deicing. The room operates at saturated conditions at the design temperature, and deicing raises the air temperature allowing condensate buildup, liquid or ice, on the now colder room components resulting in undesired release onto food products therein.

The present invention is shown in FIG. 1 and incorporates a unit cooler that provides increased dehumidification without increased complexity for the refrigeration system installer or operator. The invention achieves humidity levels below 90% in refrigerated rooms.

An integrated refrigeration and dehumidification unit cooler system 10 according to an embodiment of the invention is shown in FIGS. 1 and 2. Therein the system 10 provides both refrigerated and dehumidified air for controlling temperature and humidity in a refrigerated room 11, such as a food processing room. The refrigeration system 10 comprises a refrigeration compressor 12 that delivers refrigerant as a hot, high pressure discharge gas from an outlet through outlet line 14, to the inlet of a condenser 16. The condenser 16 changes the refrigerant from a hot gas to a warm liquid by rejecting heat from the refrigerant into another medium such as air or water. The compressor 12 and condenser 16 are located exterior of the room, generally as outdoor units. The warm liquid is conveyed to a unit cooler 18 located in the room 11 from an outlet of the condenser 16 through refrigerant line 20.

As shown in FIG. 2, the unit cooler 18 includes a housing 22 typically mounted at the ceiling area 23 of the room. The housing 20 encloses an expansion device 24, a refrigeration DX coil 26, and a subcool/reheat SR coil 28. The inlet of the SR coil 28 is connected to line 20. The outlet of the SR coil 28 is connected to the expansion device 24. The expansion device 24 is connected to the inlet of the DX coil 26. The coils are thus directly fluidly connected. The outlet of the DX coil 26 is connected by line 34 to the inlet or suction side of the compressor 12.

The unit cooler 18 includes a fan 36 that draws return air from an inlet 38 in an air path serially through the coils 26, 28 to an outlet 40 for circulating conditioned supply air as indicated by the arrows and return to the inlet 32. As hereinafter described, the supply air is at a supply air temperature for maintaining temperature conditions at or below a design temperature and at a dew point preventing condensation on overhead surfaces notwithstanding defrost cycles and operational conditions.

The SR coil 28 uses coil air from DX coil 26 to subcool the refrigerant liquid to a temperature below saturation. The subcooled refrigerant liquid is moved through subcooled liquid line 30 to the expansion device 24, which receives refrigerant liquid in a sub-cooled state. With sub-cooled refrigerant liquid, the expansion device can do more cooling than it could with warm, saturated refrigerant liquid in the prior art system. The sub-cooling of the refrigerant liquid gives DX coil 26 the ability to do more cooling than it could with warm, saturated refrigerant liquid. The extra cooling capacity allows DX coil 26 to cool air entering the unit cooler to a lower temperature than it could with warm, saturated refrigerant liquid. In this case the colder air coming from DX coil 26 is still at 100% Rh. Once the DX coil 26 cools the air to this new lower level, the air is heated by SR coil 28. In this way the air ends up with a relative humidity below 100% Rh due to the reheating by the SR coil 28 to a temperature above the moisture saturation condition.

The unit cooler 18 refrigerates and dehumidifies the air by delivering air to the room 11 at a temperature that is still low enough to hold the room at the design temperature, but at a lower relative humidity. Obtainable relative humidity numbers for air leaving the SR coil 28 are in the range of 75% to 90%.

Because SR coil 28 drives lower air temperatures in DX coil 26, the temperature that unit cooler 18 delivers to room 11 is nominally the same after SR coil 28 as it would be in the prior art DX refrigeration system of FIG. 2. The heat that SR coil 28 removes from the refrigeration system results in colder air leaving DX coil 26. Then SR coil 28 returns the same amount of heat to the treated air, an air dehumidification process that is termed overcool and reheat.

The differences between the present and prior art systems are apparent from the pressure enthalpy diagram of FIG. 5 that represents the specific energy states and corresponding pressures in the refrigeration cycle. Therein, the prior art system is shown in solid lines with the component numbers used in FIG. 3. The present system is shown in solid and dashed lines with the component numbers used in FIG. 1. The prior art refrigeration system comprises the line 110 that represents the refrigeration compressor that delivers hot, high pressure refrigerant discharge gas through a point that the inlet to the condenser. The line 114 represents the condenser that changes the refrigerant from a hot gas to a warm liquid by rejecting heat from the refrigerant into another medium such as air or water. The warm liquid is conveyed to the inlet of the expansion device. The line 118 represents the expansion device that expands the refrigerant from a warm, high pressure liquid to a cold, low pressure mixture of liquid and vapor refrigerant. This expanded refrigerant is delivered to the inlet of the DX coil located in a unit cooler. The line 122 represents the change in enthalpy across the DX Coil where the resulting cold refrigerant vapor is conveyed at low pressure to the inlet of the compressor whereat the process repeats.

The enthalpy diagram for the present invention, as in prior art system, the line 12 represents the change across the compressor and the line 14 the change across the condenser. The contribution of the SR coil to subcooling is denoted by line 28 wherein the refrigerant liquid is further cooled into the lower enthalpy sub-cooled zone. The change across the expansion device is denoted by line 18. Thus, the contribution of the DX coil to the change in enthalpy is increased as denoted by line 26 in comparison to line 122.

The contribution of the SR coil 28 to the dehumidification of supply air to the room is shown in the comparison of the systems in the psychometric charts of FIGS. 6 and 7. FIG. 6 shows the prior art system of FIG. 3 and represents the specific energy states and corresponding absolute moisture content of air. The air is cooled in the unit cooler by the DX coil along line 122 and exits at the outlet point 132. The air passes through the room along line 102 and returns to the unit cooler at inlet point 130 with a relative humidity of 85 to 98%. As the air passes through DX coil 122 sensible heat (dry) and latent heat (wet) are removed. This has the effect of lowering the dry bulb temperature of the air and lowering the absolute moisture content of the air. The air exits at outlet point 132 with a relative humidity at or near 100%. This air motion cools the refrigerated room while gaining sensible heat, which increases the dry bulb temperature, and gaining latent heat, which increases the moisture content. In this process moisture is removed from the air, but the room stays in a condition of high relative humidity. This creates an environment where temperature fluctuation and refrigeration system defrost cycles can create dew point temperatures that are higher than the temperature of equipment or structural components in the refrigerated room. That is when liquid water condenses on the overhead equipment and/or the overhead structure. The condensed water then drips down upon the storage area or production area in the refrigerated room. The air is returned at point 132 around a 98% Rh.

In FIG. 1 and 2, the present invention, the SR coil 28 reduces the dry bulb temperature to the outlet point 40 providing an 81% Rh. The air circulation in the room along line 11 results in an intake condition at inlet point 38 of 85% Rh, a humidity condition that accommodates room temperature excursions during operation. This is achieved by the unit cooler delivering colder, lower humidity air. Although the air is gaining sensible heat, which increases the dry bulb temperature, and gaining latent heat, which increases the moisture content, the room remains in a condition of low enough relative humidity to prevent condensation dripping. This creates an environment where temperature fluctuation and refrigeration system defrost cycles do not create dew point temperatures that are higher than the temperature of equipment or structural components in the refrigerated room.

The method for establishing the above operation for a refrigerated space will initially prescribe a supply air temperature for maintaining an upper permissible room temperature or less and also the required air flow volume. Based on operations within the room, a dew point range for the supply air is prescribed that will provide acceptable humidity conditions preventing condensation on overhead surfaces. The dew point thus establishes the operating dew point for the DX coil and the temperature rise from the SR coil. A compressor/condenser package is then selected for achieving the dew point. A SR coil size is determined to achieve the temperature rise at the air flow. The heat balance of the system is then determined. Because of the addition of the SR coil and consequent lowering of the fluid temperature to the expansion device and DX coil, a lower DX coil dew point may result requiring reiterations of these steps be undertaken as necessary to establish the interface between the coils to achieve performance within the design limits.

An example of the foregoing advantages is set forth in the following example.

EXAMPLE

A prior art system is operated with a compressor, Model No. 06DM316, from Caryle, providing a saturated suction temperature (SST) of 25° F. and a saturated condensing temperature (SCT) of 115° F. The net refrigeration effect of this refrigeration system is (nominal 4 Tons) 48,000 btu/hr of cooling, which is delivered to a refrigerated room through a system of FIG. 3 using a traditional unit cooler. The power consumption of the refrigeration compressor is 5.5 kW in this condition. The unit cooler will treat air from the refrigerated space while following the saturation line. Accordingly, although moisture is removed, the air is delivered to the refrigerated room at 100% RH. The unit cooler has in intake of 2,640 scfm of air at 40.0° F. db/39.8° F. wb (98% RH). The unit cooler supplies air to the refrigerated space at 31.1° F. db/31.0° F. wb (99% RH). The unit cooler removes (nominal) 21,000 btu/hr of latent (wet) cooling load from the refrigerated space. The unit cooler removes moisture removal at (nominal) 17 lbs/hr.

A system in accordance with FIG. 1 uses a compressor, Model number 06DA818 from Caryle Corp, which is slightly larger than the compressor in the prior example. The compressor operates at a saturated suction temperature (SST) of 20° F. (lower than the prior example) and a saturated condensing temperature (SCT) of 115° F. (same as the example). The net refrigeration effect of this refrigeration system is (nominal 5.1 Tons) 61,000 btu/hr of cooling, which is delivered to a refrigerated room through a traditional unit cooler. The power consumption of the refrigeration compressor is 5.8 kW (5.5% higher than the base example) in this condition of (nominal 5.2 Tons) 61,000 btu/hr of cooling. The unit cooler will draw in 2,640 scfm of air at 40.0° F. db/38.1° F. wb (85% RH). This unit cooler supplies air to the refrigerated space at 31.2° F. db/29.3° F. wb (81% RH). The unit cooler removes (nominal) 21,000 btu/hr of latent (wet) cooling load from the refrigerated space. This is the same latent cooling as the prior base example, but with a lower relative humidity than the base example, by 14% RH. The unit cooler moisture removal is (nominal) 17 lbs/hr, which is the same as the base example.

These examples demonstrate that the system in accordance with the present invention incorporating the integral sub-cool/reheat coil able to achieve dehumidification in a refrigerated space runs with 5.5% more power consumption than traditional DX refrigeration, but with a lower relative humidity by 14% RH. Such a reduction accommodates the temperature excursions typically encountered in food processing without presenting a condensation condition on in-place equipment. The modest power increase is substantially less, both in capital and operating cost, that a supplemental installation. The synergistic effects of the subcooling and reheating of the SR coil thus provides an effective solution for overcoming condensation problems in refrigerated storage applications.

Having thus described a presently preferred embodiment of the present invention, it will now be appreciated that the objects of the invention have been fully achieved, and it will be understood by those skilled in the art that many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the present invention. The disclosures and description herein are intended to be illustrative and are not in any sense limiting of the invention, which is defined solely in accordance with the following claims. 

1. A unit cooler dehumidification system for maintaining a design temperature of below about 41° F. in a refrigerated space with supply air at a humidity and at a dew point temperature preventing condensation during operation, said system comprising: a compressor located exterior of the space, said compressor having a suction side to which a working fluid is supplied as a vapor at a saturated suction temperature and a discharge side from which the working fluid is discharged as a vapor at a high pressure and elevated temperature; a condenser heat exchanger located exterior of the space, said condenser heat exchanger supplied with said superheated vapor from said compressor for exhausting heat from said vapor and discharging the working fluid as a saturated liquid at high pressure; a unit cooler located entirely in the space, said unit cooler having a fan for providing an air flow passage between an inlet receiving return air from said room and an outlet delivering the supply air to said room, a cooling coil in said air flow passage registering with said inlet for cooling and dehumidifying said return air to a temperature below said design temperature to said dew point temperature and a heating coil in said air flow passage downstream of said coiling coil and registering with said outlet for heating the cooled return air from said cooling coil to a temperature at about said design temperature, said heating coil directly supplied with said liquid at high pressure from said condenser heat exchanger and discharging said liquid at a reduced temperature; an expansion device directly supplied with said liquid at a reduced temperature from said heating coil and supplying said liquid at reduced pressure to an inlet of said cooling coil, said cooling coil discharging said liquid at reduced pressure from an outlet directly and continuously to said inlet of said compressor.
 2. The system as recited in claim 1 wherein said saturated suction temperature of said compressor is sufficient to attain said dew point temperature at said cooling coil and a relative humidity of 70% to 85%.
 3. The system as recited in claim 2 wherein said saturated suction temperature is below 25° F.
 4. The system as recited in claim 3 wherein said heating coil has about 15% to 35% of the heat transfer surface of the cooling coil.
 5. The system as recited in claim 4 wherein said heating coil has about 20% to 30% of the heat transfer surface of the cooling coil.
 6. The system as recited in claim 2 wherein said dew point temperature is below about 25° F.
 7. The system as recited in claim 2 wherein said supply air temperature is about 31° F.
 8. A method of operating a unit cooler contained in a refrigerated room at a room design temperature below 41° F. under conditions avoiding condensation on overhead surfaces of the room, comprising the steps of: selecting a dew point for supply air that avoids the condensation and a supply air temperature for maintaining said room design temperature; providing a unit cooler having a refrigeration coil and a heating coil serially disposed in a fan assisted air passage to routing return air to an inlet and the supply air from an outlet; providing a first working fluid flow path between an outlet of the refrigeration coil and the heating coil having a compressor and a heat exchanger; providing a second working fluid flow path directly serially connecting an outlet of said heating coil and an inlet of said cooling coil; providing an expansion device in said second working fluid flow path; operating said heating coil at a temperature to achieve said dew point; and operating said heating coil to achieve said supply temperature while enabling said heating coil to achieve said dew point temperature.
 9. The method as recited in claim 8 including the step of providing said heating coil with about 15% to 35% as much heat transfer area as said cooling coil.
 10. The method as recited in claim 9 including the step of providing said heating coil with about 20% to 30% as much heat transfer area as said cooling coil.
 11. The method as recited in claim 10 including operating said compressor as a suction temperature sufficient for the cooling coil to attain said dew point temperature and said heating coil to attain said supply air temperature.
 12. A unit cooler for mounting at the ceiling area of a refrigerated space comprising: a housing having an inlet and an outlet; means for mounting said housing at the ceiling area of the refrigerated space; a fan in said housing for establishing an air flow between said inlet and said outlet; a coiling coil in said housing adjacent said inlet having a cooling heat transfer surface; a heating coil disposed in said housing between said cooling coil and said outlet whereby said air flow is directed serially from said cooling coil to said heating coil; first conduit means for directly fluidly connecting an inlet of said heating coil with a source of high pressure fluid at an elevated temperature whereby said heating coil is effective for heating the air flow from said cooling coil and lowering the temperature of said high pressure fluid; second conduit means in said housing for directly and continuously operatively fluidly connecting an outlet of said heating coil with an inlet of said cooling coil; an expansion device in said second conduit means for converting said high pressure fluid from said outlet of said heating coil to a low pressure cooled vapor at a cooled temperature for supply to an outlet of said heating coil; and third conduit means for directly fluidly connecting an outlet of said cooling coil to the inlet of a compressor, wherein said heating heat transfer surface has an effective area of about 15% to 35% of said cooling heat transfer surface.
 13. The unit cooler as recited in claim 12 wherein said heating heat transfer surface has an effective area of about 20% to 30% of said cooling heat transfer surface 